1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and particularly to the field of marine seismic data processing. More particularly, the invention relates to noise attenuation in dual sensor towed marine seismic streamers.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downwardly into the subsurface of the earth. In a marine seismic survey, the seismic signal will first travel downwardly through a body of water overlying the subsurface of the earth.
Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflections are detected by seismic sensors at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data is recorded and processed to yield information relating to the geologic structure and properties of the subterranean formations and their potential hydrocarbon content.
Appropriate energy sources for seismic surveys may include explosives or vibrators on land and air guns or marine vibrators in water. Appropriate types of seismic sensors may include particle motion sensors in land surveys and water pressure sensors in marine surveys. Particle motion sensors are typically particle velocity sensors, but particle displacement, particle acceleration sensors, or pressure gradient sensors may be used instead of particle velocity sensors. Particle velocity sensors are commonly known in the art as geophones and water pressure sensors are commonly known in the art as hydrophones. Both seismic sources and seismic sensors may be deployed by themselves or, more commonly, in arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers have been proposed that contain water particle velocity sensors such as geophones or particle acceleration sensors such as accelerometers, in addition to hydrophones. The pressure sensors and particle motion sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable.
U.S. patent application Ser. No. 11/021,919, entitled “Motion Sensors in a Towed Streamer”, filed on Dec. 22, 2004, and assigned to an affiliated company of the assignee of the present invention describes an example of particle motion sensors appropriate to place in a marine seismic streamer in combination with pressure sensors.
The sources and streamers are submerged in the water, with the seismic sources typically at a depth of 5-15 meters below the water surface and the seismic streamers typically at a depth of 5-40 meters. Seismic data gathering operations are becoming progressively more complex, as more sources and streamers are being employed. These source and streamer systems are typically positioned astern of and to the side of the line of travel of the seismic vessel. Position control devices such as depth controllers, paravanes, and tail buoys are used to regulate and monitor the movement of the seismic streamers.
Alternatively, the seismic cables are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water. In this alternative case, the source may be towed behind a vessel to generate acoustic energy at varying locations, or the source may also be maintained in a stationary position.
Recorded seismic data contains signal in terms of the useful primary reflections as well as much noise. The recorded noise may be coherent (that is, acts like a traveling wave) or random. Examples of coherent noise include ground roll, guided waves, side-scattered noise, cable noise, air wave, power lines, and multiples. Multiples are especially strong relative to primaries in marine seismic surveys, because the water-earth and, particularly, the air-water interfaces are strong seismic reflectors due to their high acoustic impedance contrasts. Examples of random noise include poorly planted geophones, wind motion, transient movements in the vicinity of the cable, wave motion in the water causing the cable to vibrate, and electrical noise from the recording instruments. Seismic surveying employing towed marine seismic streamers has a large number of noise sources to deal with.
Marine seismic streamers are typically divided into streamer sections approximately 100 meters in length, and can extend to a length of thousands of meters. A typical streamer section includes an external jacket, strength members, spacers, an electrical wire bundle, and connectors. The external jacket protects the interior of the streamer section from water ingress. The strength members, usually two or more, run down the length of each streamer section from end connector to connector, providing axial mechanical strength. The spacers maintain the cylindrical shape of the streamer section. The electrical wire bundle also runs down the length of each streamer section, and includes electrical power conductors and electrical data communication wires. In some instances, fiber optic connections for data communication are included in the wire bundle. Connectors at the ends of each streamer section link the section mechanically, electrically or optically to adjacent sections and, hence, ultimately to the seismic towing vessel.
The external jacket and strength members are typically designed to be straight and uniformly thick throughout the length of the streamer section while they are not subject to external stress. The electrical wire bundle, on the other hand, is typically designed to be capable of deforming, so that it can bend readily as the streamer cable itself bends while turning in the water, or is being deployed from or retrieved onto the streamer winch for storage on the survey vessel.
Sensors, typically hydrophones or arrays of hydrophones, along with geophones or arrays of geophones, are located within the streamer. The hydrophones record the pressure wavefield and the geophones record the particle velocity wavefield. The sensors have sometimes been located within the spacers for protection. The distance between spacers is normally about 0.7 meters. An array of sensors, typically comprising 8 or 16 hydrophones and collocated geophones, normally extends for a length of about 6.25 meters or 12.5 meters, respectively. These two lengths allow 16 or 8 arrays, respectively, in a standard 100 meter seismic section.
The interior of the seismic streamers is typically filled with a core material to provide buoyancy and desirable acoustic properties. For many years, most seismic streamers have been filled with a fluid core material. A drawback to using fluid-filled streamer sections is the noise generated by vibrations as the streamer is towed through the water. These vibrations develop internal pressure waves traveling through the fluid in the streamer sections, which are often referred to as “bulge waves” or “breathing waves”.
Several approaches have been employed to reduce the bulge wave noise problem in fluid-filled steamer sections. For example, one approach is the use of stretch sections at the front and rear of the seismic streamer. Another approach is the application of low-cut frequency filters. Another approach is to introduce compartment blocks in the sections to impede the vibration-caused bulge waves from traveling continuously along the streamer. Another approach is to introduce open cell foam into the interior cavity of the streamer section. The open cell foam restricts the flow of the fluid fill material in response to the transient pressure change and causes the energy to be dissipated into the external jacket and the foam over a shorter distance. Yet another approach is to eliminate the fluid from the streamer sections, so that no medium exists in which bulge waves can develop. This approach is exemplified by the use of streamer sections filled with a solid core material or softer solid material instead of a fluid. However, in any solid type of material, some shear waves will develop, which can increase the noise detected by both the hydrophones and the geophones. Shear waves cannot develop in a fluid fill material since fluids have no shear modulus. Additionally, many conventional solid core materials are not acoustically transparent to the desired pressure waves.
Mechanical energy may travel in a “fast” mode, typically as a longitudinal wave along the strength members, at about 1100-1300 meters/second or in a “slow” mode, typically as a transverse wave, at about 20-60 meters/second. This fast mode is the dominant mechanical noise encountered with hydrophones, while the slow mode is the dominant mechanical noise encountered with geophones.
Another approach to address the noise problem is to combine several hydrophones into an array (also known as a group) to attenuate a slow moving wave. Typically, an equal number of hydrophones are positioned between or on both sides of the spacers so that pairs of hydrophones sense equal and opposite pressure changes. Summing the hydrophone signals from an array can then cancel out some of the noise.
Many of the conventional methods apply seismic processing to pressure sensors only. However, the pressure sensor data has spectral notches caused by the water surface reflections, commonly referred to as sea surface ghosts. These spectral notches are often in the seismic acquisition frequency band. Thus, the usable portion of the pressure sensor data is frequency band limited away from the spectral notches and cannot cover the entire seismic acquisition frequency band. This limitation can be avoided by using both pressure sensors and particle motion sensors in a “dual-sensor” streamer.
L. Amundsen and A. Reitan, in their article “Decomposition of multicomponent sea-floor data into upgoing and downgoing P- and S-waves”, Geophysics, Vol. 60, No. 2, March-April, 1995, p. 563-572, describe a method for deghosting dual sensor cable data in the water layer and on the sea floor. Amundsen and Reitan construct a decomposition filter to apply to pressure recorded by hydrophones just above the sea floor and the radial and vertical components of the particle velocity recorded by geophones just below the sea floor. The decomposition filter separates the data into upgoing and downgoing P- and S-waves, yielding the deghosted wavefield in the up-going components. The decomposition filter coefficients depend upon the P- and S-wave velocities and the density at the sea floor.
U.S. patent application Ser. No. 10/935,515, entitled “System for Attenuation of Water Bottom Multiples in Seismic Data Recorded by Pressure Sensors and Particle Motion Sensors”, filed on Sep. 7, 2004, and assigned to an affiliated company of the assignee of the present invention describes a method for attenuation of water bottom multiples in marine seismic data. The method includes calculating up-going and down-going wavefield components from pressure sensor and particle motion sensor signals, extrapolating the wavefields to the water bottom, and utilizing the extrapolated wavefields and a water bottom reflection coefficient to generate an up-going wavefield substantially without water bottom multiples.
Several other methods known in the art provide procedures for processing dual sensor data to reduce the ghost notches in marine seismic data acquired utilizing towed marine streamers, ocean bottom cable, or vertical cable. These processing methods utilize pressure sensor data and vertical particle motion sensor data to construct filters that separate the dual sensor data into up-going and down-going wavefield components. The up-going wavefield component is the deghosted wavefield. As is well known in the art, particle motion sensors are significantly more sensitive to mechanical noise than pressure sensors. Thus, the deghosting approaches of these prior procedures are introducing additional noise into the up-going pressure wavefield component.
Thus, a need exists for a method of deghosting which effectively attenuates the infiltration of particle motion sensor noise from the separated pressure wavefields. In particular, a need exists for a method that can remove the receiver-side ghost and attenuate the mechanical particle motion sensor noise in a towed streamer.